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<p align="center"><b><font size="4">Synoptic scale features of the tropospheric 
  circulation over tropical<br>
  South America during the WETAMC TRMM/LBA experiment<br>
  </font></b> </p>
<p align="center"><font size="2">N. J. FERREIRA, A. A. CORREIA and M. C. V. RAM&Iacute;REZ</font><br>
  Instituto Nacional de Pesquisas Espaciais<br>
  <font size="2">C. P. 515, 12210-010</font> S&atilde;o Jos&eacute; dos Campos, 
  SP, Brazil</p>
<p align="center"><br>
  Received February 5, 2002; accepted September 1, 2003</p>
<p align="center"><br>
  RESUMEN</p>
<p align="justify">Este trabajo analiza las caracter&iacute;sticas sin&oacute;pticas 
  de la circulaci&oacute;n de los niveles altos y bajos de la troposfera sobre 
  Am&eacute;rica del Sur, durante la primera campa&ntilde;a para colecta de datos 
  atmosf&eacute;ricos en mesoescala en la &eacute;poca h&uacute;meda (Atmospheric 
  Mesoscale Campaign in the Wet Season - WETAMC) del Experimento de Gran Escala 
  en la Biosfera-Atm&oacute;sfera de la Amazonia (LBA). Esta campa&ntilde;a fue 
  realizada entre los meses de enero y febrero de 1999 simult&aacute;neamente 
  con la validaci&oacute;n de los datos del sat&eacute;lite Tropical Rainfall 
  Measuring Mission (TRMM) sobre Brasil. Fueron utilizados datos puntuales reticulares 
  de los National Centers for Environmental Predictions (NCEP) para analizar los 
  vientos y sus campos de divergencia asociados, a manera de definir los patrones 
  prevalecientes de la circulaci&oacute;n troposf&eacute;rica en la regi&oacute;n. 
  Las im&aacute;genes TRMM fueron utilizadas para delinear los patrones de lluvia. 
  Los resultados muestran que ocurrieron por lo menos cuatro patrones diferentes 
  en los campos de viento en la alta troposfera, durante el per&iacute;odo estudiado. 
  Estos patrones est&aacute;n asociados, principalmente, a la circulaci&oacute;n 
  anticicl&oacute;nica de Bolivia y a los v&oacute;rtices cicl&oacute;nicos en 
  el noreste de Brasil. Los resultados tambi&eacute;n indican que, en general, 
  la actividad convectiva sobre extensas &aacute;reas de Am&eacute;rica del Sur 
  es din&aacute;micamente sustentada por divergencias a niveles altos. Dos de 
  los seis v&oacute;rtices observados, presentaron desplazamientos an&oacute;malos 
  en la Amazonia durante el per&iacute;odo de estudio. En los niveles bajos de 
  la Zona de Convergencia del Atl&aacute;ntico Sur (ZCAS) no hubo actividad, a 
  excepci&oacute;n de la primera quincena de enero; adicionalmente, incursiones 
  en direcci&oacute;n ecuatorial de sistemas frontales de latitud media modularon 
  la actividad convectiva en la regi&oacute;n occidental de la Amazonia. Durante 
  el per&iacute;odo de estudio, el flujo convergente de vapor de agua de nivel 
  bajo derivado de los datos del NCEP present&oacute; un patr&oacute;n parcialmente 
  consistente con los campos de lluvia del TRMM.</p>
<p align="center">ABSTRACT</p>
<p align="justify">This paper analyzes the synoptic scale features of the upper 
  and lower level tropospheric circulation over tropical South America during 
  the first Atmospheric Mesoscale Campaign in the Wet Season (WETAMC) of the Large 
  Scale Biosphere &#8211; Atmosphere Experiment in Amazonia (LBA). This campaign 
  occurred in January and February, 1999 and concomitantly with the validation 
  of the Tropical Rainfall Measuring Mission (TRMM) satellite over Brazil. National 
  Centers for Environmental Predictions (NCEP) grid point data were used to analyze 
  the winds and its associated divergence fields and define the prevailing patterns 
  of the tropospheric circulation in that region. TRMM images were used to depict 
  the rain patterns. The results show at least four distinct patterns in the upper 
  air winds fields during the WETAMC-TRMM/LBA, involving mainly the Bolivian anticyclonic 
  circulation and cyclonic vortices in the vicinity of Northeast Brazil. The results 
  also suggest that, in general, the convective activity over vast areas of South 
  America is dynamically supported by upper level divergence. Two out of a total 
  of six vortices were observed to have anomalous displacements into the Amazonia 
  during the study period. At lower levels, the South Atlantic Convergence Zone 
  (SACZ), except for the first half of January, was not active but equatorward 
  incursions of midlatitude frontal systems disturbed the convective activy in 
  the Southwestern Amazon basin. Besides, during the period of the study, the 
  low-level moisture divergence as derived from NCEP data does not show convergence 
  where the TRMM rain field clearly reveals the existence of precipitation areas.</p>
<p align="justify"><b>Key words:</b> Cyclonic vortices, TRMM, South 
  Atlantic convergence zone, Bolivian High</p>
<p align="justify"><br>
  <b>1. Introduction</b><br>
  The first Atmospheric Mesoscale Campaign in the Wet Season (WETAMC) of the Large 
  Scale Biosphere &#8211; Atmosphere Experiment in Amazonia (LBA) (Silva <i>Dias 
  et al.,</i> 2002) occurred in Rondonia, Brazil during the southern summer, 
  from January to February, 1999. This season was chosen because typically it 
  is characterized by a pronounced tropospheric circulation variability with great 
  impact on the regional rainfall regimen. Rutledge <i>et al.</i> (2000) noticed 
  that during the WETAMC, the development of the mesoscale convective systems 
  (MCSs) occurred under two distinct meteorological regimens: monsoon and break. 
  In the former case, low level westerly/northwesterly winds prevailed with humidity 
  relatively large and abundant precipitation; in the latter, the convective activity 
  was under low-level easterly flow, with a much drier troposphere. In addition, 
  Cifelli <i>et al.</i> (2002), suggest that the distinct wind regimens (easterly 
  and westerly) observed in Rondonia during the WETAMC control the local rainfall 
  and are associated with the displacements of frontal systems in Southeastern 
  Brazil.</p>
<p align="justify"> Several studies have shown that many of the large scale characteristics 
  of the circulation over South America can be simulated using three dimensional 
  and time varying tropical heat sources (Silva <i>Dias et al., </i>1983; Gandu 
  and Gesiler, 1991) as well as with a forcing due to the extratropical convection 
  (Belassiano, 2000). Gandu and Silva Dias (1998) showed that during the austral 
  summer both the convective activity over the western and central Pacific as 
  well as over Africa have an impact on the high level tropospheric circulation 
  and the subsidence patterns in the middle troposphere over South America and 
  adjacent oceans. Thus, from the synoptic point of view, the tropospheric circulation 
  over tropical South America and neighboring areas is more easily understood 
  by considering the dynamics of the upper and lower troposphere separately. In 
  this context the main features of the upper level circulation in this region 
  are the interhemispheric bifurcation of the flow over the Eastern Pacific, the 
  anticyclonic circulation known as the Bolivian High (BH) (Virji, 1981), the 
  upper level cyclonic vortices in the vicinity of Northeast Brazil (CVNE) (Kousky 
  and Gan, 1981; Mishra <i>et al.,</i> 2001), the troughs originated from the 
  Southern Hemisphere mid-latitudes (TRs), the tropical troughs over the Northern 
  and Southern Pacific Ocean and the troughs over the Northern Atlantic Ocean 
  (Ram&iacute;rez <i>et al.,</i> 1999). On the other hand, at lower levels the 
  outstanding features include the South Atlantic Convergence Zone (SACZ) (Kodama, 
  1992; Carvalho <i>et al.,</i> 2002), the convective activity over the Amazonia 
  (Greco <i>et al.,</i> 1990) and the stable semi-arid climate of Northeast 
  Brazil (Rao and Hada, 1999).</p>
<p align="justify"> During the WETAMC-TRMM/LBA experiment the main scientific 
  interest was on the mesoscale features of the regional circulation. As a consequence, 
  there was a significant effort addressed to understanding the local interactions 
  involving the biosphere/atmosphere, cloud dynamics and microphysics, TRMM-satellite 
  precipitation radar and micrometeorology (e.g., Cifelli <i>et al.,</i> 2002; 
  Petersen and Rutledge, 2001). However, it was deemed that a diagnostic study 
  of the most important synoptic scale features of the atmospheric circulation 
  during the WETAMC-TRMM/LBA experiment would certainly be useful to acquire a 
  better understanding of the acting mesoscale processes. Thus this work is concerned 
  with the dominant synoptic aspects of the atmospheric circulation over tropical 
  South America during that time period. Emphasis was given on the diagnosis of 
  the upper and lower tropospheric winds and the analysis of the TRMM-derived 
  rainfall fields.</p>
<p align="justify"><b><br>
  2. Data and methodology</b><br>
  The variables used in this study were the daily air temperature (<i>T</i>), 
  zonal (<i>u</i>) and meridional (<i>v</i>) wind components and specific 
  humidity (<i>q</i>) obtained from the global model reanalysis project NCEP/NCAR 
  (National Centers for Environmental Prediction/National Center for Atmospheric 
  Research) (Kalnay <i>et al.,</i> 1996). They have a 2.5&deg; x 2.5&deg; lat 
  &#8211; lon resolution and are given at 1000-, 925-, 850-, 700-, 500-, 250-, 
  200- and 100-hPa isobaric levels. The spatial domain extends from 15&deg;N to 
  4&deg;S and 95&deg;W to 15&deg;W and the studied period includes the months 
  of January and February, for which monthly means (using data from 1978 to 1999) 
  and daily analyses during the WETAMC&#8211;TRMM/LBA period were analyzed. The 
  moisture flux and wind divergence were also examined. The first variable is 
  given in spherical coordinates by</p>
<p align="center"><img src="/img/revistas/atm/v17n1/a02pag15.jpg"></p>
<p align="justify">where:<br>
  <img src="/img/revistas/atm/v17n1/a02pag15b.gif"> is the wind, <i>a</i> is the 
  Earth&#8217;s mean radius, and &#955; and &#966; are the latitude and longitude, 
  respectively.<br>
  The software GrADS &#8211; Grid Analysis and Display System (Doty, 1997) was 
  used for processing and graphical visualization of the results. A script was 
  also used to plot CVNE trajectories reading the daily lat-lon vortex center 
  coordinates from an ASCII file with the GrADS &#8220;read&#8221; scripting command, 
  and appropriate commands to analyze and plot the data over a map background.</p>
<p align="justify">The analyses of rainfall fields over South America were based 
  on gridded daily rainfall data obtained using the &#8220;3B42-TRMM- Adjusted 
  Merged-Infrared Precipitation&#8221; algorithm which calibrates VIRS (Visible 
  and Infrared Sounder) infrared measurements with precipitation estimates given 
  by TMI (TRMM Microwave Imager). The calibration parameters were then used to 
  obtain a daily composition of IR data from the available meteorological geostationary 
  satellites. The algorithm yields estimates of precipitation near the surface 
  with a 1&deg; x 1&deg; lat - lon resolution, within the tropical belt between 
  40&deg; N and 40&deg; S. Details of this methodology are described in Huffman 
  <i>et al.</i> (1995) and in the User&acute;s Guide, made available by the 
  TRMM Data and Information System (TSDIS) at <a href="http://tsdis.gsfc.nasa.gov" target="_blank">http://tsdis.gsfc.nasa.gov</a>. The 
  3B42 data were obtained from NASA (National Aeronautics and Space Administration) 
  via File Transfer Protocol (FTP), at the following address: http://daac.gsfc.nasa.gov/.</p>
<p align="justify"><b>3. Results</b><br>
  Next a brief description of the climatological characteristics of the upper 
  and lower levels tropospheric circulation over tropical South America during 
  the first two months of the austral summer is presented. The dominant features 
  are compared with those of January/February 1999, the period during which the 
  WETAMC &#8211; TRMM / LBA experiment was carried out.</p>
<p align="justify"><i>3.1. Upper level features</i><br>
  The upper level circulation in tropical South America in January (<a href="#fig1a">Fig. 
  1a</a>) is governed by BH and CVNE. Strong divergent areas are observed in the 
  Southeastern Atlantic Ocean, associated with the SACZ and along the Inter-Tropical 
  Convergence Zone (ITCZ). The most pronounced areas of divergence over Brazil 
  are induced by the BH/CVNE circulation system. Since the convective activity 
  in these areas is strong (not shown), the divergence may be responsible for 
  the organization of the rainfall in Amazonia and Central and Southeastern Brazil. 
  Therefore, eventual displacements of the BH/CVNE system would imply shifts of 
  the convective activity. Furthermore, the observed differences in the divergence 
  fields over the continent and Southwestern Atlantic Ocean suggest that the SACZ 
  dynamics may be modulated by distinct processes over these regions. On the other 
  hand, during January 1999 (<a href="#fig1a">Fig. 1b</a>), the 250 hPa flow in 
  tropical South America showed a slightly different pattern. Instead of the typical 
  anticyclonic circulation center and the associated downstream trough, one observes 
  a pair of anticyclones and the CVNE. The anticyclonic circulation over western 
  Bolivia is the BH and the other downstream anticyclonic circulation (AT) appears 
  when BH extends eastward. <i>Ram&iacute;rez et al.</i> (1999) suggest that this 
  type of situation favors the inland displacement of CVNE and is associated with 
  the presence of the SACZ or the passage of troughs in Southeastern Brazil. It 
  becomes evident from the comparison between the climatological pattern and the 
  January 1999 conditions (<a href="#fig1a">Fig. 1b</a>) that most of the tropospheric 
  circulation in South and Southeastern Brazil was under the influence of the 
  AT, thus with anomalous characteristics. A closed circulation, instead of a 
  trough, was the dominant feature over the Tropical Atlantic Ocean near NE, due 
  to the high frequency of occurrence of vortices in that region.<br>
</p>
<p align="center"><a name="fig1a"></a> <img src="/img/revistas/atm/v17n1/a02pag17.gif"></p>
<p align="center">Fig. 1. Streamlines and horizontal divergence (1 x 10<sup><font size="2">-6</font></sup> 
  s<sup><font size="2">-1</font></sup>) of the mean 250 hPa wind for <br>
  (a) Januay, 1978-1999 and (b) January, 1999.</p>
<p align="justify"><br>
  Except for a zone of weaker divergence along the SACZ, the mean upper tropospheric 
  circulation in February, 1999 (<a href="#fig2">Fig. 2a</a>) is quite similar 
  to that of the preceding month. On the other hand, during the WETAMC &#8211; 
  TRMM/LBA (<a href="#fig2">Fig. 2b</a>) one can notice that the BH had shifted 
  slightly southward and the trough near NE, not yet clearly defined, had a smaller 
  meridional amplitude with a southeastward orientation. The observed divergence 
  field suggests weak convective activity along the SACZ, but not along the ITCZ 
  over the tropical Western Atlantic Ocean.</p>
<p align="center"><a name="fig2"></a><img src="/img/revistas/atm/v17n1/a02pag17b.gif"></p>
<p align="center"> Fig. 2. &nbsp;As in <a href="#fig1a">Fig.1</a>, but for (a) 
  February, 1978-1999 and (b) February, 1999.</p>
<p align="justify"><i><br>
  3.2. Lower level features</i><br>
  The mean 850 hPa circulation over tropical South America during the austral 
  summer is dominated by the trade winds at low latitudes and the subtropical 
  South Atlantic Ocean High (SAH). Under this circumstance, the trade winds have 
  an important role in advecting moisture from the tropical Atlantic Ocean into 
  NE and Amazonia (Rao and Marques, 1984). This moisture feeds the convective 
  activity over land. On the other hand, the northeasterly winds in the occidental 
  sector of the SAH are important for establishing and maintaining the SACZ&acute;s 
  oceanic branch.</p>
<p align="justify"> The mean 850 hPa water vapor flux divergence fields during 
  January and February (<a href="#fig3">Figs. 3a</a>, <a href="#fig4">4a</a>) 
  show high values along the northern/northeastern coast of Brazil. Moisture convergence 
  is also observed along portions of the SACZ, the slopes of the Andes in Southwestern 
  Bolivia (topographic effect), western NE and over the Brazilian Amazonia. The 
  main observed feature of the lower level circulation during WETAMC &#8211; TRMM/LBA 
  (Figs. <a href="#fig3">3b</a>, <a href="#fig4">4b</a>) is the vast region of 
  moisture convergence in the Amazonia and along the SACZ, although other centers 
  were also observed to the west of NE and Central Amazonia. The 850 hPa moisture 
  flux convergence was slightly pronounced, extending from the southern part of 
  the Amazonia to the Southeastern Atlantic Ocean (due to the passage of frontal 
  systems), despite the absence of SACZ events in February 1999.<br>
</p>
<p align="center"><a name="fig3"></a><img src="/img/revistas/atm/v17n1/a02pag18.gif"> 
</p>
<p align="center">Fig. 3. &nbsp;Mean 850 hPa streamlines and horizontal moisture flux 
  divergence (1 x 10<sup><font size="2">-5</font></sup> gKg<sup><font size="2">-1</font></sup>s<sup><font size="2">-1</font></sup>) 
  for<br>
  (a) January, 1978-1999 and (b) January, 1999.</p>
<p align="center"><br>
  <a name="fig4"></a><img src="/img/revistas/atm/v17n1/a02pag19.gif"></p>
<p align="center">Fig. 4. &nbsp;As in <a href="#fig3">Fig.3</a> , but for (a) 
  February, 1978-1999 and (b) February, 1999.</p>
<p align="justify"><br>
  The moisture flux convergence patterns over Amazonia and Southeastern and Northern 
  Brazil during January and February 1999 are partly consistent with the rainfall 
  field retrieved from TRMM data (Figs. <a href="#fig5">5a-b</a>). For several 
  areas (e.g. the eastern slopes of the Andes Mountains near Southwestern Bolivia, 
  ITCZ, the north/northeast Brazilian coast, western portions of NE Brazil and 
  the Southwestern Atlantic Ocean) NCEP data do not show convergence where the 
  satellites indicates the existence of precipitating areas. It is likely that 
  this disagreement may be associated with the model&#8217;s space resolution, 
  parametrization schemes and the representativeness of the assimilated data. 
  It should be pointed out that the conventional meteorological network over tropical 
  South America is quite sparse.<br>
  <br>
</p>
<p align="center"><a name="fig5"></a><img src="/img/revistas/atm/v17n1/a02pag19b.gif"></p>
<p align="center">Fig. 5. 3B42 Rinfall (mm) retrieved from TRMM data for (a) January, 
  1999 and (b) February, 1999.</p>
<p align="justify"><br>
  A Hovm&ouml;ller diagram of the daily 3B42 TRMM rainfall data (<a href="#fig6">Fig. 
  6</a>), from Southern Amazonia to the Southwestern Atlantic Ocean (following 
  SACZ axis), during January and February 1999 reveals that: a) the SACZ was present 
  only during the period of the 6<sup>th</sup>-18<sup>th</sup> of January when 
  the rainfall was heavier (about 60 mm) between 44&deg;W and 48&deg;W, over S&atilde;o 
  Paulo state and b) SACZ episodes were not observed in February although there 
  were two situations of heavy rainfall due to the penetration of frontal systems 
  (Feb. 18 and Feb. 26), along the coast of S&atilde;o Paulo state. In all these 
  events, frontal displacements in Southeastern Brazil prompted rainfall in Rondonia 
  and Southern Amazonia, in agreement with the findings of Rickenback <i>et al. 
  </i>(2001), and Cifelli <i>et al.</i> (2001). As they suggested the large scale 
  modulates the convective activity in Rondonia, so that low-level westerly (easterly) 
  winds tend to yield larger (smaller) areas of weak (strong) precipitation.</p>
<p align="center"><br>
  <a name="fig6"></a><img src="/img/revistas/atm/v17n1/a02pag20.gif"></p>
<p align="center">Fig. 6.&nbsp; Hovm&ouml;ller diagram of the daily rainfall data (mm) 
  derived from TRMM data, from Southern Amazonia (5&deg; S, 60&deg;W)
  to Southwestern Alantic Ocean (-30&deg; S, 30&deg;W), during January 1<sup><font size="2">st</font></sup> 
  to February 28<sup><font size="2">th</font></sup>, 1999.</p>
<p align="justify"><i><br>
  3.3 Upper level circulation patterns during the WETAMC&#8211;TRMM/LBA</i><br>
  The upper level circulation during the WETAMC&#8211;TRMM/LBA was characterized 
  by BH, CVNE, troughs and an anticyclonic system (AT) located between Southeastern 
  Brazil and adjacent oceanic areas. The BH position was observed to be quite 
  variable, oscillating between 90&deg;W and 55&deg;W. This displacement was associated 
  with CVNEs moving inland, the formation of AT and equatorward incursion of middle 
  latitude troughs (TRs). Six major events of CVNEs (<a href="#fig7">Fig.7</a>) 
  were observed during the studied period, with a mean life time of approximately 
  6.9 days and most of them showing a vertical extension between 200 and 400 hPa.</p>
<p align="justify">The first vortex (V1) appeared on January 2<sup>nd</sup>, 1999 
  at approximately 12&deg;S and 30&deg;W, over the NE eastern coast and gradually 
  shifted toward the Amazonia where by January 13<sup>th</sup> it had dissipated. 
  During this movement, its associated subsidence and sensible heat flux were 
  responsible for an evident suppression of the convective activity over the Amazonia. 
  Concomitantly, the BH had also shifted westward reaching the Southeastern Pacific 
  Ocean (Southwestern Per&uacute;). The displacement of V1 into Amazonia may indeed 
  be regarded as an anomalous situation, for just a few percent of the vortices 
  originating in the NE region move inland (Ram&iacute;rez <i>et al.,</i> 1999).</p>
<p align="justify"> The second vortex (V2) had a shorter life time (3 days); it 
  originated between the coast of Rio Grande do Norte state and the Equator, on 
  January 14<sup>th</sup>, 1999. It also moved westward reaching Northeastern 
  Par&aacute; state where it dissipated on the 17<sup>th</sup>. The origin of 
  V2 was associated with a southwestern amplification of a ridge, in its turn, 
  to an anticyclonic circulation over Northwestern Africa. The genesis of vortices 
  over the NE, linked to this kind of circulation was studied by Paix&atilde;o 
  and Gandu (2000). During the active stage of V2, the BH was found in the southwest 
  of Per&uacute;, over the Pacific Ocean, and connected to the anticyclonic circulation 
  of the Southwestern Atlantic Ocean. The atmospheric circulation pattern during 
  this period affected the convective activity over tropical Brazil.</p>
<p align="center"><br>
  <a name="fig7"></a><img src="/img/revistas/atm/v17n1/a02pag21.gif"></p>
<p align="center">&nbsp;</p>
<p align="center">Fig. 7. &nbsp;Trajectories of the CVNEs during the WETAMA-TRMM/LBA 
  Experiment for<br>
  (a) January, 1999 and (b) February, 1999.</p>
<p align="justify"><br>
  The third vortex (V3), contrasting to the usual genesis, originated over the 
  continent, 13&deg;S, 49.5&deg;W (north of Goi&aacute;s state) on January 20<sup>th</sup>, 
  1999 and shifted eastward reaching the Sergipe state (eastern coast of NE) by 
  January 25<sup>th</sup>, 1999 where it dissipated. During this period, the BH 
  moved eastward, toward Bolivia and the AT position was over the Southern Paran&aacute; 
  state (Southern Brazil).</p>
<p align="justify"> The fourth vortex (V4) originated over the eastern coast of 
  NE, approximately 12&deg;S on January 29<sup>th</sup>, 1999. This vortex had 
  a westward displacement, reaching the northern part of Rondonia state, where 
  it dissipated. At the same time the BH was between Southern Paraguay and Northern 
  Argentina, thus further south of its climatogical position. This is a situation 
  that potentially could decrease rainfall along the trajectory of the vortex 
  center.</p>
<p align="justify"> The fifth vortex (V5) originated over the Atlantic Ocean (18&deg;S, 
  20&deg;W ) on February 1<sup>st</sup>, 1999. This vortex also moved westward 
  and dissipated seven days later over the Southern Piaui state (northern NE). 
  The BH was located during this event over Northern Argentina, further south 
  than its climatological position. Finally, the sixth vortex (V6) appeared at 
  the eastern side of Southern Bahia state on February 13<sup>th</sup>, 1999, 
  due to the expansion of a trough to the east of the BH. This vortex had a westward 
  displacement, dissipating over the northern part of Minas Gerais state (Southeastern 
  Brazil) nine days later.<br>
  <br>
  An analysis of the prevailing upper circulation patterns over tropical South 
  America during the WETAMC&#8211;TRMM/LBA period is presented next. The observed 
  four main patterns are quite distinct, regarding the relative positons of the 
  BH and CVNEs.</p>
<p align="justify"><i>3.3.1 BH/CVNE circulation pattern</i><br>
  The BH/CVNE circulation pattern dominated the first days of the first, fourth 
  and sixth events of CVNE. Under this kind of situation, the vortex geneses is 
  due to the amplification of an upper level ridge associated with equatorward-moving 
  cold fronts over South America (Kousky and Gan, 1981). The BH is well defined 
  and is situated over the Southwestern Amazonia near Bolivia, and the CVNEs are 
  found over the Southwestern Atlantic Ocean (<a href="#fig8">Fig. 8</a>). By 
  the beginning of January 1999, the rainfall associated to the BH/CVNE circulation 
  extended from the Southern Amazonia to the Southwestern Atlantic Ocean due to 
  a cold front in the region. The highest daily rainfall values (30 to 40 mm) 
  occurred in Southeastern Brazil where the cold front advanced into the states 
  of Minas Gerais and Rio de Janeiro (<a href="#fig9">Fig. 9</a>). Fair weather 
  was observed in the NE when the CVNE was still located over the ocean. As mentioned 
  before, the rainfall at this time of the year seems to be associated with the 
  presence of upper level divergence in the transition region between the two 
  opposite circulation systems: the BH and the CVNE. Therefore, eventual oscillations 
  of the BH/CVNE system (not shown) are accompanied by northeastward/southeastward 
  displacements of frontal systems in Southeastern and Southern Brazil.<br>
</p>
<p align="center"><a name="fig8"></a><img src="/img/revistas/atm/v17n1/a02pag23.gif"></p>
<p align="center">&nbsp;</p>
<p align="center">Fig. 8. 250 hPa streamlines for January 2<sup><font size="2">nd</font></sup>, 
  1999, associated to the BH/CVNE circulation pattern.</p>
<p align="center"><br>
  <a name="fig9"></a><img src="/img/revistas/atm/v17n1/a02pag23b.gif"></p>
<p align="center">Fig. 9. 3B42 rainfall (mm) retrieved from TRMN data, associated 
  to the BH/CVNE circulation pattern for <br>
  January 2<sup><font size="2">nd</font></sup>, 1999.</p>
<p align="justify"><i><br>
  3.3.2 BH /CVNE/AT circulation pattern</i><br>
  This type of pattern, identified by Ram&iacute;rez <i>et al.</i> (1999), is 
  more evident when the CVNE is in its intensification stage and during its displacement 
  inland. In this situation the BH generally acquires a northwest/southeast orientation 
  as the CVNE initiates its movement. The BH elongates over Southeastern Brazil 
  as the CVNE moves inland (<a href="#fig10">Fig. 10</a>). Depending on the intensification 
  of the ridge, the BH may split into two parts, with the eastern flank forming 
  a new anticyclonic center (AT) over Southeastern Brazil and adjacent oceanic 
  areas.</p>
<p align="center"><a name="fig10"></a><img src="/img/revistas/atm/v17n1/a02pag24.gif"></p>
<p align="center">Fig.10.&nbsp;250 hPa streamlines for January 7<sup><font size="2">th</font></sup>, 
  1999, associated to the BH/CVNE/AT circulation pattern.</p>
<p align="justify"><br>
  The amplification of the ridge along the southeastern side of the BH for the 
  V1 event mentioned, occurred with the arrival of a frontal system from Argentina, 
  which remained stationary for five days (Jan. 6&#8211;10) and established the 
  SACZ. As a consequence, warmer air advection in the lower levels contributed 
  to the intensification of the upper level ridge, creating favorable conditions 
  for enhancing the convection in Northern S&atilde;o Paulo state and Western 
  Amazonia (<a href="#fig11">Fig. 11</a>). During this period, the ridge amplification 
  favored the formation of an AT to the south of the CVNE, thus establishing the 
  circulation pattern involving the BH (with a westward shift toward the Southern 
  Pacific Ocean), the CVNE (moving into the continent) and the AT. Although the 
  SACZ is a dominant feature in the lower levels of the summer circulation over 
  Brazil, only one SACZ episode was observed during the January&#8211;February 
  1999 period. </p>
<p align="justify"> Related to the BH/CVNE/AT pattern, it was observed that despite 
  the existence of a vortex, a new one may form to the northeast of the AT, as 
  exemplified by V2, V4 and V6. Ram&iacute;rez <i>et al. </i>(1999) performed 
  a climatological study on the formation mechanisms of tropical vortices and 
  concluded that 27 % of the CVNEs are associated with the occurrence of ATs. 
  During this type of situation, the release of latent heat from stationary frontal 
  systems or the SACZ would maintain the AT and the associated upper level region 
  of divergence.</p>
<p align="justify"> Whenever there is an intensification of the southeastern sector 
  of the BH over Southeastern Brazil and adjacent oceanic areas, the spatial rainfall 
  pattern resembles a convective cloud band with an NW&#8211;SE orientation to 
  the west of the ridge and a mid-latitude trough in the upper levels (<a href="#fig9">Fig. 
  9</a>). This rainfall band is generally associated with a frontal system. Once 
  the cloudiness of the western and northern sectors of the CVNE reaches the NE 
  coast, it interacts with the convective activity associated with the BH and 
  frontal systems, frequently forming a Y pattern of cloudiness (Ferreira <i>et 
  al.,</i> 2001). The Y pattern follows the CVNE during its inland displacement.<br>
</p>
<p align="center"><a name="fig11"></a><img src="/img/revistas/atm/v17n1/a02pag25.gif"></p>
<p align="center">Fig. 11. 3B42 rainfall (mm) retrieved from TRMM data, associated 
  to the BH/CVNE circulation pattern, <br>
  for January 7<sup><font size="2">th</font></sup>, 1999.</p>
<p align="justify"><i><br>
  3.3.3 BH/AT circulation pattern</i><br>
  The establishment of the BH/AT pattern, which lasts a few days, is generally 
  observed after the dissipation of a CVNE over the central continent. The observed 
  circulation between Jan. 18&#8211;19 (<a href="#fig12">Fig. 12</a>) is a good 
  example of this type of situation, when the BH moves westward off the Peruvian 
  coast and the AT is found over the Southwestern Atlantic Ocean near Southeastern 
  Brazil. In addition, a trough forms between the two anticyclonic circulations 
  over Argentina, favoring the convection in Southeastern Paraguay. In this case, 
  the prevailing rainfall pattern consists of a convective band to the southwest 
  of the AT and stratiform rainfall over vast areas in the Amazonia (<a href="#fig13">Fig. 
  13</a>).<br>
</p>
<p align="center"><a name="fig12"></a><img src="/img/revistas/atm/v17n1/a02pag26.gif"></p>
<p align="center">Fig. 12. 250 hPa streamlines for January 18<sup><font size="2">th</font></sup>, 
  1999, associated to the BH/AT circulation pattern.</p>
<p align="justify">&nbsp;</p>
<p align="center"><a name="fig13"></a><img src="/img/revistas/atm/v17n1/a02pag26b.gif"></p>
<p align="center">Fig. 13. 3B42 rainfall (mm) retrieved from TRMM data, associated 
  to the BH/AT circulation pattern,<br>
  for January 18<sup><font size="2">th</font></sup>, 1999.</p>
<p align="justify"><i><br>
  3.3.4 BH/TR circulation pattern</i><br>
  Two episodes involving BH and TR were observed during the WETAMC-TRMM/LBA experiment. 
  The first one occurred between Feb. 11 &#8211; 12 when the BH showed an almost 
  symmetric configuration extending zonally as far as Goi&aacute;s state (central 
  Brazil), with its core centered near Northern Chile (<a href="#fig14">Fig.14</a>). 
  A pronounced mid-latitude trough stretching from Northern S&atilde;o Paulo state 
  to the Southwestern Atlantic Ocean (see marked line, <a href="#fig14">Fig 14</a>), 
  was associated to a band of moderate rainfall produced by a frontal system (<a href="#fig15">Fig. 
  15</a>). Under this situation, the CVNE is not present and the frontal system 
  is free to move northeastward. Similarly, during the second episode (Feb. 23 
  &#8211; 28) the BH was over Northern Chile, favoring the penetration of two 
  mid-latitude troughs in Southern Brazil (not shown). As mentioned earlier, this 
  pattern synchronizes the displacements of the rainfall band northeastward in 
  Southeastern Brazil and southwestward in Amazonia. In other words, by the time 
  it reaches Southeastern Brazil, the frontal system interacts with the convection 
  in Amazonia, organizing and making it shift northeastward. This feature also 
  favors the formation of troughs with a &#8220;NW/SE&#8221; inclination over 
  the Atlantic Ocean, near the eastern coast of the NE and the formation of AT 
  in central Western Brazil.<br>
</p>
<p align="center"><a name="fig14"></a><img src="/img/revistas/atm/v17n1/a02pag27.gif"></p>
<p align="center">Fig. 14. 250 hPa stremlines for January 12<sup><font size="2">th</font></sup>, 
  1999, associated to the BH/TR circulation pattern.</p>
<p align="center"><br>
  <a name="fig15"></a><img src="/img/revistas/atm/v17n1/a02pag28.gif"></p>
<p align="center">Fig. 15. 3B42 rainfall (mm) retrieved from TRMM data, associated 
  to the BH/TR circulation pattern,<br>
  for January 12<sup><font size="2">th</font></sup>, 1999.</p>
<p align="justify"><b><br>
  4. Concluding remarks</b><br>
  This study analyzes the synoptic scale features of the upper and lower levels 
  tropical circulation over South America during the first Atmospheric Mesoscale 
  Campaign in the Wet Season (WETAMC) of the Large Scale Biosphere &#8211; Atmosphere 
  Experiment in Amazonia (LBA). The analyses were carried out using the National 
  Centers for Environmental Predictions (NCEP) data and rainfall fields derived 
  from TRMM (Tropical Rainfall Measuring Mission) data. During the studied period 
  (January and February 1999) the tropospheric circulation in the region showed 
  distinct features from the climatological pattern. At least four distinct flow 
  patterns were identified in the upper troposphere, mainly related to the Bolivian 
  High (BH) and cyclonic vortices in the vicinity of Northeast Brazil (CVNEs). 
  In two observed events, CVNEs moved from the NE coast into Amazonia and their 
  associated subsidence and changes in the circulation pattern may have impaired 
  the precipitation in some parts of Amazonia.</p>
<p align="justify"> It was also observed that from the synoptic point of view 
  the instability in the region is dynamically supported by upper level divergence 
  in the transition zone of the Bolivian anticyclone (BH) and the cyclonic circulation 
  in the vicinity of NE Brazil (CVNE) in the upper troposphere. Kousky and Gan 
  (1981) suggested that the convective activity associated with CVNEs depends 
  on thermal direct circulation, the position of the vortex and the direction 
  of their movement. In the present study, it is suggested that the convective 
  activity depend on the interaction between the BH/CVNE circulations and not 
  only on CVNE dynamics. At lower levels moisture convergence derived from NCEP 
  data coincides only partially with the rain areas detected by TRMM satellites. 
  Also, significant interaction involving frontal systems and convective activity 
  in Southwestern Amazonia was evident.</p>
<p align="justify"><b><br>
  Acknowledgements</b><br>
  This work is part of the &#8220;Intera&ccedil;&atilde;o Biosfera &#8211; Atmosfera 
  em Mesoscala na Amaz&ocirc;nia&#8221; Project sponsored by FAPESP, Process 1997 
  /9926 &#8211; 9. The NCEP data used in this work were provided by NOAA &#8211; 
  CIRES Climate Diagnostics Center, Boulder, Colorado, USA, from the website at 
  <a href="http://www.cdc.noaa.gov/" target="_blank">http://www.cdc.noaa.gov/</a>. The TRMM data are distributed by Goddard Space Flight 
  Center (GSFC/NASA) through the website <a href="http://lake.nascom.nasa.gov/data/dataser/TRMM/" target="_blank">http://lake.nascom.nasa.gov/data/dataser/TRMM/</a> 
  and the software used for reading the satellite data is provided by TRMM Science 
  and Data Information System (TSDIS). Thanks are due to Mr. Marco A.M. Lemes 
  for going through the manuscript. We also thank the anonymous reviewers, who 
  contributed significantly to the final manuscript. The first author was partially 
  supported by the Conselho Nacional de Desenvolvimento Cient&iacute;fico e Tecnol&oacute;gico 
  under Grant number 300486/96-0.</p>
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